Certain microorganisms are capable of accumulating intracellular triacylglycerol, single-cell oil, under suitable physiological conditions. This property has been utilized for production of single-cell oils mainly for specialty use and high value products, such as cosmetics and nutritional applications. Up to date the need of single-cell oils for these purposes has been satisfied by low or moderate scale production processes in which process economy and sustainability of production have not played a significant role.
At present there is increasing interest for the use of non-fossil raw materials as alternatives for the fossil ones as precursors of transportation fuel. The main challenge appears to be that the production of single-cell oil should be carried out in such a large scale that problems typical for large scale aqueous microbial processes are evitable. Characteristically a single-cell oil production process requires high energy input and produces large aqueous side streams and single cell debris. Thus, the current technology for ultra large-scale aqueous microbial processes does not seem economically feasible and includes several problems with respect to environmental issues.
Carbohydrates are the richest single reserve of biomass on the earth and thus the only realistic substrate for large scale single-cell oil production. Lignocelluloses are the most abundant source of carbohydrates on Earth. However, lignocellulosic or other composite biomaterials require extensive processing to make them available for microorganisms as nutrients and as the source of single cell oil. Concomitantly with the processing of lignocellulosic biomass, non-fermentable side streams, such as lignin, will arise. On the other hand, the single cell oil production process itself yields cell mass and side streams, which still contain a fair amount of nutrients.
The re-utilization of the side streams of various fermentation processes and improvement of the overall process economy has been suggested in some publications in the prior art. The quantitatively dominating side streams of single-cell oil production are fermentation waste water and cell debris remaining after removal of oil from the cells. US 2009/0064567 A1 discloses the production of biological oils by heterotrophic fermentation by growing microorganism of the kingdom Stramenopile. The publication suggests the re-cycling of de-lipidated biomass or hydrolyzed biomass to media used for cultivation of the microorganism.